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The gut microbial communities of mammals provide numerous benefits to their hosts. However, given the recent development of the microbiome field, we still lack a thorough understanding of the variety of ecological and evolutionary factors that structure these communities across species. Metabarcoding is a powerful technique that allows for multiple microbial ecology questions to be investigated simultaneously. Here, we employed DNA metabarcoding techniques, predictive metagenomics, and culture-dependent techniques to inventory the gut microbial communities of several species of rodent collected from the same environment that employ different natural feeding strategies [granivorous pocket mice (Chaetodipus penicillatus); granivorous kangaroo rats (Dipodomys merriami); herbivorous woodrats (Neotoma albigula); omnivorous cactus mice (Peromyscus eremicus); and insectivorous grasshopper mice (Onychomys torridus)]. Of particular interest were shifts in gut microbial communities in rodent species with herbivorous and insectivorous diets, given the high amounts of indigestible fibers and chitinous exoskeleton in these diets, respectively. We found that herbivorous woodrats harbored the greatest microbial diversity. Granivorous pocket mice and kangaroo rats had the highest abundances of the genus Ruminococcus and highest predicted abundances of genes related to the digestion of fiber, representing potential adaptations in these species to the fiber content of seeds and the limitations to digestion given their small body size. Insectivorous grasshopper mice exhibited the greatest inter-individual variation in the membership of their microbiomes, and also exhibited the highest predicted abundances of chitin-degrading genes. Culture-based approaches identified 178 microbial isolates (primarily Bacillus and Enterococcus), with some capable of degrading cellulose and chitin. We observed several instances of strain-level diversity in these metabolic capabilities across isolates, somewhat highlighting the limitations and hidden diversity underlying DNA metabarcoding techniques. However, these methods offer power in allowing the investigation of several questions concurrently, thus enhancing our understanding of gut microbial ecology.

Lyme disease, caused by Borrelia burgdorferi sensu stricto and most commonly transmitted by Ixodes scapularis Say (Ixodida: Ixodidae), is the most common tick-borne disease in Maryland. Because B. burgdorferi s.s. is maintained in enzootic cycles among wild mice (Peromyscus spp) and Ixodes spp ticks, differing patterns of parasitism of ticks on mice could impact the infection prevalence with B. burgdorferi. We determined the infection prevalence of Peromyscus spp as well as questing and partially engorged nymphal ticks collected at six sites on private land in five counties in Maryland from May to August 2020. Questing nymph infection prevalence (NIP) was 14%. We trapped 1258 mice and collected 554 ticks and 413 ear tissue samples. The prevalence of infested Peromyscus spp varied based on host age and sex, with older and male mice more likely to be infested. We detected a significant difference amongst the proportion of attached Ixodes and the location of trapping. Similarly, the prevalence of B. burgdorferi infected Peromyscus spp mice varied between locations (average mouse infection prevalence was 40%), with the highest prevalence in locations where Ixodes were the most commonly found ticks. The B. burgdorferi infection prevalence in partially engorged I. scapularis nymphs retrieved from Peromyscus spp was ~36% which lends further support to the host infection prevalence. Local differences in distribution of infected vectors and reservoirs are important factors to consider when planning interventions to reduce Lyme disease risk.

Integrated tick management (ITM) is a comprehensive strategy used to reduce presence of ticks and their associated pathogens. Such strategies typically employ a combination of host and non-host targeted treatments which often include fipronil-based, rodent-targeted bait boxes. Bait boxes target small-bodied rodents, specifically white-footed mice (Peromyscus leucopus Rafinesque) that not only play a crucial role in the blacklegged tick (Ixodes scapularis Say (Ixodida: Ixodidae)) life cycle, but also in the transmission of numerous pathogens, primarily Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner (Spirochaetales: Spirochaetaceae), the causal agent of Lyme disease. This study aimed to determine the effect of bait box deployment configuration on tick burden reduction while also further exploring bait consumption and P. leucopus abundances as measures of bait box usage and effectiveness. Boxes were deployed on nine properties within each of six neighborhoods (n = 54) in two different configurations: grid and perimeter. Multiple factors were analyzed as potential predictors for reduction in tick burdens using a backward stepwise selection procedure. Results confirmed the perimeter configuration was a more effective deployment strategy. In addition, overall P. leucopus abundance was a significant predictor of tick burden reduction while bait consumption was not. These findings not only further support the recommended perimeter deployment configuration but provide insight into effective utilization in areas of high P. leucopus abundance. The identification of this significant relationship, in addition to configuration, can be utilized by vector control professionals and homeowners to make informed decisions on bait box placement to make sustained impacts on the I. scapularis vector and associated pathogens within an ITM framework.

Efforts to remove invasive species may benefit native species, but the effects can be complex and unpredictable. Thus, studies of invasive-species removal provide important information for guiding management and providing insight about variation in post-removal impacts within the community. Using southern pine-grassland ecosystems as a model system, we hypothesized that removal of the long-established red-imported fire ant (Solenopsis invicta, hereafter RIFA) would positively influence altricial Peromyscus species, due to increased survival of young in the nest and thus increased recruitment to the population, but would not impact semi-precocial hispid cotton rats (Sigmodon hispidus), which are mobile more quickly after birth and thus at less risk of depredation by RIFA. We compared small mammal populations on sites treated with a granular insecticide (Extinguish Plus) to remove RIFA in southwestern Georgia, United States, from April 2018 to December 2019. As expected, we detected no difference in cotton rat recruitment. However, contrary to our prediction, the same was true for cotton mice (Peromyscus gossypinus) and oldfield mice (Peromyscus polionotus). We found RIFA removal increased survival both of cotton rats and cotton mice, increasing average population rate of change (λ) on treated sites during the study period. In contrast, we observed lower survival of oldfield mice, with similar λ estimates on treated and untreated sites, but low sample sizes were problematic for this species. Our results show that removal of invasive species can have positive impacts for native species, but both the magnitude of RIFA effects on small mammals and mechanisms by which impacts occur are complex.

Determining how variation in morphology affects animal performance (and ultimately fitness) is key to understanding the complete process of evolutionary adaptation. Long tails have evolved many times in arboreal and semi-arboreal rodents; in deer mice, long tails have evolved repeatedly in populations occupying forested habitat even within a single species (Peromyscus maniculatus). Here, we use a combination of functional modeling, laboratory studies, and museum records to test hypotheses about the function of tail-length variation in deer mice. First, we use computational models, informed by museum records documenting natural variation in tail length, to test whether differences in tail morphology between forest and prairie subspecies can influence performance in behavioral contexts relevant for tail use. We find that the deer- mouse tail plays little role in statically adjusting center of mass or in correcting body pitch and yaw, but rather it can affect body roll during arboreal locomotion. In this context, we find that even intraspecific tail-length variation could result in substantial differences in how much body rotation results from equivalent tail motions (i.e., tail effectiveness), but the relationship between commonly-used metrics of tail-length variation and effectiveness is non-linear. We further test whether caudal vertebra length, number, and shape are associated with differences in how much the tail can bend to curve around narrow substrates (i.e., tail curvature) and find that, as predicted, the shape of the caudal vertebrae is associated with intervertebral bending angle across taxa. However, although forest and prairie mice typically differ in both the length and number of caudal vertebrae, we do not find evidence that this pattern is the result of a functional trade-off related to tail curvature. Together, these results highlight how even simple models can both generate and exclude hypotheses about the functional consequences of trait variation for organismal-level performance.

Warming climate and increasing desertification urge the identification of genes involved in heat and dehydration tolerance to better inform and target biodiversity conservation efforts. Comparisons among extant desert-adapted species can highlight parallel or convergent patterns of genome evolution through the identification of shared signatures of selection. We generate a chromosome-level genome assembly for the canyon mouse (Peromyscus crinitus) and test for a signature of parallel evolution by comparing signatures of selective sweeps across population-level genomic resequencing data from another congeneric desert specialist (Peromyscus eremicus) and a widely distributed habitat generalist (Peromyscus maniculatus), that may be locally adapted to arid conditions. We identify few shared candidate loci involved in desert adaptation and do not find support for a shared pattern of parallel evolution. Instead, we hypothesize divergent molecular mechanisms of desert adaptation among deer mice, potentially tied to species-specific historical demography, which may limit or enhance adaptation. We identify a number of candidate loci experiencing selective sweeps in the P. crinitus genome that are implicated in osmoregulation (Trypsin, Prostasin) and metabolic tuning (Kallikrein, eIF2-alpha kinase GCN2, APPL1/2), which may be important for accommodating hot and dry environmental conditions.

The blacklegged tick (Ixodes scapularis Say) is the primary vector of Borrelia burgdorferi sensu stricto (Spirochaetales: Spirochaetaceae), the Lyme disease agent in North America. The basic reproduction number (R0) for B. burgdorferi in I. scapularis in the Northeast is highly sensitive to the probability that engorged larvae survive the winter, molt into nymphs, and find a host. These processes are dependent on local environmental variables, including climate, host population size and movement, and tick behavior. A simple model is presented for estimating host-finding success from the ratio of tick abundance in two subsequent years, accounting for overwinter survival and possible differences in host associations between nymphs and larvae. This model was parameterized using data from two sites in mainland Connecticut and two on Block Island, RI. Host abundance and tick burdens were estimated via mark–recapture trapping of the primary host, Peromyscus leucopus Rafinesque. Overwintering survival was estimated using engorged larvae placed in field enclosures at each site. Only nymphs were recovered alive, and no significant differences in model parameters were observed between Connecticut and Block Island. Host-finding success was predicted to be high across a wide range of host association patterns at three of four sites. Assuming equivalent host association between larvae and nymphs, R0 was also estimated to be greater than one at three of four sites, suggesting these conditions allow for the persistence of B. burgdorferi. The model output was highly sensitive to differences between nymphal and larval host associations.

Urbanization is a selective force that is known to drive changes in the population dynamics of wildlife. How animals adapt to changing environmental conditions is crucial to their survival in these environments. Relative brain size, or cranial capacity, is a known proxy of behavioral flexibility, and can be used to assess how well a species has adapted to a particular environment. We examined changes in cranial capacity in a time series of small mammal skulls collected from urban and rural populations in southwestern Pennsylvania. Skulls from urban populations were collected from Allegheny County, an area that experienced rapid urbanization over the past century, and skulls of rural populations from the Powdermill Nature Reserve of the Carnegie Museum, which has remained relatively unchanged forest over the same period. Our results show that Peromyscus leucopus and Microtus pennsylvanicus from urban populations had significantly greater cranial capacity than their rural counterparts, but the opposite was true for Eptesicus fuscus. We found no difference in relative cranial capacity across time in any of the small mammal species. Our results suggest that a larger cranial capacity is selected for in an urban environment and reinforces the hypothesis that behavioral flexibility is important for animals to adapt to novel environments.

Although Peromyscus leucopus (deermouse) is not considered a genetic model system, its genus is well suited for addressing several questions of biologist interest, including the genetic bases of longevity, behavior, physiology, adaptation, and it’s ability to serve as a disease vector. Here we explore a diversity outbred approach for dissecting complex traits in Peromyscus leucopus, a non-traditional genetic model system. We take advantage of a closed colony of deer-mice founded from 38 individuals between 1982 and 1985 and subsequently maintained for 35+ years (∼40-60 generations). From 405 low-pass (∼1X) short-read sequenced deermice we accurately imputed genotypes at 17,751,882 SNPs. Conditional on observed genotypes for a subset of 297 individuals, simulations were conducted in which a QTL contributes 5% to a complex trait under three different genetic models. The power of either a haplotype- or marker-based statistical test was estimated to be 15-25% to detect the hidden QTL. Although modest, this power estimate is consistent with that of DO/HS mice and rat experiments for an experiment with ∼300 individuals. This limitation in QTL detection is mostly associated with the stringent significance threshold required to hold the genome-wide false positive rate low, as in all cases we observe considerable linkage signal at the location of simulated QTL, suggesting a larger panel would exhibit greater power. For the subset of cases where a QTL was detected, localization ability appeared very desirable at ∼1-2Mb. We finally carried out a GWAS on a demonstration trait, bleeding time. No tests exceeded the threshold for genome-wide significance, but one of four suggestive regions co-localizes with Von Willebrand factor. Our work suggests that complex traits can be dissected in founders-unknown P. leucopus colony mice in much the same manner as founders-known DO/HS mice and rats, with genotypes obtained from low pass sequencing data. Our results further suggest that the DO/HS approach can be powerfully extended to any system in which a founders-unknown closed colony has been maintained for several dozen generations.

The Peromyscus maniculatus species complex is one of the most widespread group of small mammals in North America. However, the taxonomy and phylogenetic relationships among its constituent taxa remain unclear. As part of a revision of Peromyscus specimens from the highlands of the Trans-Mexican Volcanic Belt in central Mexico, we identified five individuals collected in 1968 that differed externally from other Peromyscus specimens, although morphologically similar to P. labecula and P. melanotis, both latter in the P. maniculatus species complex. Based on cranial measurements and mitochondrial DNA sequences, we aimed to more accurately determine the phylogenetic relationships and the taxonomic status of these individuals. Molecular phylogenetic analyses showed that the specimens formed a monophyletic clade sister to the P. maniculatus species complex. Pairwise genetic distances between those specimens and other species within the P. maniculatus species complex were greater than 7.91%. In addition, morphological analyses clearly distinguished the test specimens from P. melanotis and P. labecula. Based on the results of our molecular and morphological analyses, we conclude that these specimens represent an undescribed species of the P. maniculatus species complex, which we describe herein.

The genus Peromyscus represents a rapidly diverged clade of Cricetid rodents that contains multiple cryptic species and has a propensity for morphologic conservation across its members. The unresolved relationships in previously proposed phylogenies reflect a suspected rapid adaptive radiation. To identify functional groups of genes that may be important in reproductive isolation in a reoccurring fashion across the Peromyscus phylogeny, liver and testis transcriptomes from four species (P. attwateri, P. boylii, P. leucopus, and P. maniculatus) were generated and differential expression (DE) tests were conducted. Taxa were selected to represent members diverged from a common ancestor: P. attwateri + P. boylii (clade A), and P. leucopus + P. maniculatus (clade B). Comparison of clades (A vs. B) suggested that 252 transcripts had significant DE in the liver data set, whereas significant DE was identified for 657 transcripts in the testis data set. Further, 45 genes had DE isoforms in the 657 testis transcripts and most of these functioned in major reproductive roles such as acrosome assembly, spermatogenesis, and cell cycle processes (meiosis). DE transcripts in the liver mapped to more broad gene ontology terms (metabolic processes, catabolic processes, response to chemical, and regulatory processes), and DE transcripts in the testis mapped to gene ontology terms associated with reproductive processes, such as meiosis, sperm motility, acrosome assembly, and sperm–egg fusion. These results suggest that a suite of genes that conduct similar functions in the testes may be responsible for the adaptive radiation events and potential reoccurring speciation of Peromyscus in terms of reproduction through varying expression levels.

The blacklegged tick, Ixodes scapularis Say, is the primary Lyme disease vector in the eastern United States. Both immature stages of I. scapularis take blood meals from mice belonging to the genus Peromyscus. Mice are active during the night and spend the majority of diel periods in nests. Thus, immature I. scapularis have a greater opportunity to drop from Peromyscus hosts while in nests compared with the forest floor. Here, we collected 11 Peromyscus nests during a 3-mo period during which the immature I. scapularis are known to be active. We then examined nesting materials for the presence of I. scapularis. Immature I. scapularis were detected in 64% of Peromyscus nests examined. Additionally, 55% of the nests contained at least one Dermacentor variabilis Say larva. Eighty-seven percent of all larval ticks found within nests were blood-fed. Because Peromyscus spp. are highly competent reservoirs of numerous tick-borne pathogens, the ticks that detach in their nests may be important for the maintenance of tick-borne diseases. However, further studies are needed to determine the fate of the I. scapularis that detach in Peromyscus nests.

Peromyscus melanophrys is a Mexican endemic distributed in seasonal tropical forests and semiarid lands. Molecular work based on mitochondrial and nuclear markers proposed the existence of four haplogroups within P. melanophrys. Peromyscus mekisturus (Puebla deer mouse) was included in one of these haplogroups. We tested the consistency between this hypothesis and external morphology, quantitative and qualitative cranial attributes, and ecological data for a sample of 1,155 specimens spanning the species distribution. We found ecological and morphological consistency with the phylogenetic pattern for P. melanophrys but not for P. mekisturus. We reassessed the taxonomic and nomenclatural status of the populations and type specimens formerly included in P. melanophrys. We concluded that these populations constitute four species: P. zamorae (Zamora deer mouse), distributed in the Mexican Plateau at elevations > 1,500 m; P. micropus (small-footed deer mouse), from lowland tropical deciduous forests of the San Pedro-Mezquital and Lerma-Santiago basins in western Mexico; P. melanophrys (black-eyed deer mouse) from lowland tropical deciduous forests of southeastern Mexico as far as eastern Oaxaca; and P. leucurus (Tehuantepec deer mouse), partially sympatric with P. melanophrys but reaching as far as Chiapas. Data on P. mekisturus were contradictory, and thus was kept as a valid species.Peromyscus melanophrys es un roedor endémico de México, distribuido en bosques tropicales caducifolios y regiones semiáridas. Datos moleculares usando marcadores mitocondriales y nucleares indicaron la existencia de cuatro haplogrupos dentro de P. melanophrys. Peromyscus mekisturus (ratón de Puebla) se incluyó dentro de uno de éstos. Con base en una muestra de 1,155 ejemplares de toda la distribución de la especie, se examinó si esta hipótesis era consistente con datos de morfología externa y craneal, morfometría e información ecológica. Se encontró coincidencia para P. melanophrys pero no para P. mekisturus. Estos resultados hicieron necesaria la reevaluación del estado taxonómico y nomenclatural de las poblaciones incluidas en P. melanophrys. Se concluye que existen cuatro especies dentro de lo que se conocía como P. melanophrys: P. zamorae (ratón de Zamora) distribuido en el Altiplano Mexicano a elevaciones ≥ 1,500 m; P. micropus (ratón de pata pequeña) distribuido en los bosques tropicales caducifolios de las cuencas San Pedro-Mezquital y Lerma-Santiago en el occidente de México; P. melanophrys (ratón de ojo oscuro) de los bosques y matorrales tropicales caducifolios del sureste de México, al menos hasta el este de Oaxaca, y P. leucurus (ratón de Tehuantepec) parcialmente simpátrico con P. melanophrys pero llegando hasta el estado de Chiapas. La información disponible no permitió asignar a P. mekisturus a ninguno de los grupos y por tanto se sigue considerando como especie válida.

Analysis of variation in whole-animal performance can shed light on causal connections between specific traits, integrated physiological capacities, and Darwinian fitness. Here, we review and synthesize information on naturally occurring variation in physiological performance capacities and how it relates to environmental adaptation in deer mice (Peromyscus maniculatus). We discuss how evolved changes in aerobic exercise capacity and thermogenic capacity have contributed to adaptation to high elevations. Comparative work on deer mice at high and low elevations has revealed evolved differences in aerobic performance capacities in hypoxia. Highland deer mice have consistently higher aerobic performance capacities under hypoxia relative to lowland natives, consistent with the idea that it is beneficial to have a higher maximal metabolic rate (as measured by the maximal rate of O2 consumption, VO2max) in an environment characterized by lower air temperatures and lower O2 availability. Observed differences in aerobic performance capacities between highland and lowland deer mice stem from changes in numerous subordinate traits that alter the flux capacity of the O2-transport system, the oxidative capacity of tissue mitochondria, and the relationship between O2 consumption and ATP synthesis. Many such changes in physiological phenotype are associated with hypoxia-induced changes in gene expression. Research on natural variation in whole-animal performance forms a nexus between physiological ecology and evolutionary biology that requires insight into the natural history of the study species.

Many organisms worldwide are responding to rapid climate change by shifting their geographic ranges. The white-footed mouse, Peromyscus leucopus, has expanded its range northward in Michigan and Wisconsin since 1980 and is now common in localities where it was previously unknown. To investigate the origin and history of the newly established populations, complete D-loop sequences were analyzed from 595 white-footed mice collected throughout the northern Great Lakes region. Mice from Wisconsin and the western Upper Peninsula (UP) of Michigan make up a well-differentiated lineage, while the eastern UP and the Lower Peninsula (LP) of Michigan form a second lineage. The two lineages diverged about 34,000 BP, well before they migrated to the Great Lakes region. The close genetic relationship between mice in the LP and those in the eastern UP is probably due primarily to accidental transport by humans and may have implications for the spread of Lyme disease in the Upper Midwest.

Animal populations can react to climate change by adapting to selection pressures, dispersing to new habitats, or undergoing plastic development at the individual level. These processes interact to form a net response, but it is difficult to determine the relative contributions of each process. In central and northern Michigan, two distinct mitochondrial lineages of Peromyscus leucopus (Upper Peninsula [UP] and Lower Peninsula [LP]) appear to have responded to climate change differently; one lineage (UP) has migrated into new geographic areas, while the other (LP) cannot. Six Michigan populations were sampled using genetic and geometric morphometric methods to test the effects of expansion versus nonexpansion demographic histories on genetic diversity and phenotypic variation. In pairwise population comparisons, there were no differences in genetic population structure or diversity between LP and UP populations or between older and newly established populations. However, the morphology of the cranium and especially the mandible were localized to each population, although moderate overlaps between the crania of UP populations and between the mandibles of Cheboygan (LP) and Menominee (UP) mice were observed. The results indicate that the UP lineage has rapidly dispersed into new areas without losing nuclear genetic diversity and that the morphological differences displayed among the populations are most likely due to plastic responses to local conditions.

Determining the environmental and ecological drivers of variation in mammalian life histories is essential for effectively monitoring responses of these taxa to global change. We investigated relationships between climate and an integral life history trait (litter size) in the geographically widespread North American deer mouse (Peromyscus maniculatus). Using a spatiotemporally comprehensive data set of P. maniculatus embryo counts assembled from digitized museum specimens and small mammal census records (> 2,000 observations spanning more than a century), we modeled the associations of litter size with short- and long-term climate variables, and tested for shifts in litter size in response to recent climate change. Litter size in P. maniculatus was correlated with length of the frost-free period, suggesting the former trait may have evolved partly as a response to abiotic or resource-driven constraints on the number of annual breeding opportunities. Short-term climate variables (temperature and precipitation during breeding) were significant predictors of litter size in some models; however, all short-and long-term climate variables other than number of frost-free days added marginal predictive power. Further analyses confirmed age-structured breeding in the most seasonal regions, where maternal body sizes and litter sizes both decrease within a year, although maternal body size itself was poorly predictive of litter size in these populations. We also found a statistically significant decrease in P. maniculatus litter size through time and across known frost-free period regimes (pre- and post-1980), as predicted if longer frost-free periods are increasing lifetime breeding opportunities in this species, although more work is required to confirm this trend and identify underlying mechanisms. Our study refines understanding of climate impacts on P. maniculatus life history and reiterates the great potential of digitized specimen data to help address outstanding questions in mammalian life history and climate change response.

Arthropods pests are most frequently associated with both plants and vertebrate animals. Ticks, in particular the blacklegged ticks Ixodes scapularis Say and Ixodes pacificus Cooley & Kohls (Acari: Ixodidae), are associated with wildlife hosts and are the primary vectors of Lyme disease, the most frequently reported vector-borne disease in the United States. Immature blacklegged ticks in the eastern United States frequently use small mammals from the genus Peromyscus as hosts. These mice are competent reservoirs for Borrelia burgdorferi, the causative agent of Lyme disease, as well as other tick-borne pathogens. To conduct surveillance on immature ticks and pathogen circulation in hosts, capture and handling of these small mammals is required. While protocols for rearing and pest surveillance on plants are common, there are very few protocols aimed at entomologists to conduct research on vertebrate–arthropod relationships. The goal of this manuscript is to provide a practical template for trapping Peromyscus spp. for vector and vector-borne pathogen surveillance and ecology for professionals that may not have a background in wildlife research. Important considerations are highlighted when targeting P. leucopus Rafinesque and P. maniculatus Wagner. Specifically, for tick and tick-borne disease-related projects, materials that may be required are suggested and references and other resources for researchers beginning a trapping study are provided.

Aerobic performance is tied to fitness as it influences an animal’s ability to find food, escape predators, or survive extreme conditions. At high altitude, where low O2 availability and persistent cold prevail, maximum metabolic heat production (thermogenesis) is an aerobic performance trait that is intimately linked to survival. Understanding how thermogenesis evolves to enhance survival at high altitude will yield insight into the links between physiology, performance, and fitness. Recent work in deer mice (Peromyscus maniculatus) has shown that adult mice native to high-altitude have higher thermogenic capacities under hypoxia compared to lowland conspecifics, but developing high-altitude pups delay the onset of thermogenesis. This suggests that natural selection on thermogenic capacity varies across life stages. To determine the mechanistic cause of this ontogenetic delay, we analyzed the transcriptomes of thermo-effector organs – brown adipose tissue and skeletal muscle – in developing deer mice native to low- and high-altitude. We demonstrate that the developmental delay in thermogenesis is associated with adaptive shifts in the expression of genes involved in nervous system development, fuel/O2 supply, and oxidative metabolism gene pathways. Our results demonstrate that selection has modified the developmental trajectory of the thermoregulatory system at high altitude and has done so by acting on the regulatory systems that control the maturation of thermo-effector tissues. We suggest that the cold and hypoxic conditions of high altitude may force a resource allocation trade-off, whereby limited energy is allocated to developmental processes such as growth, versus active thermogenesis during early development.

Borrelia mayonii is a newly described member of the Borrelia burgdorferi sensu lato complex that is vectored by the black-legged tick (Ixodes scapularis Say) and a cause of Lyme disease in Minnesota and Wisconsin. Vertebrate reservoir hosts involved in the enzootic maintenance of B. mayonii have not yet been identified. Here, we describe the first isolation of B. mayonii from naturally infected white-footed mice (Peromyscus leucopus Rafinesque) and an American red squirrel (Tamiasciurus hudsonicus Erxleben) from Minnesota, thus implicating these species as potential reservoir hosts for this newly described spirochete.

Research on the potential causes of population fluctuations in small mammals generally has focused more on biotic than abiotic factors. In this study, we asked whether the rate of population change of 2 widespread small rodents in deciduous forests of North America are associated with a set of weather events expected to affect the rodents either directly or indirectly. We analyzed a long-term (1995–2016) livetrapping data set from 6 field plots in southeastern New York, estimated rates of population change for white-footed mice and eastern chipmunks, and asked whether these rates are associated with any of a set of 5 weather variables selected a priori. Rates of population change of white-footed mice (Peromyscus leucopus), but not of chipmunks (Tamias striatus), were significantly lower during hurricane than non-hurricane years and in summers with drought than in non-drought summers. Reduced population growth of white-footed mice occurred during the same or subsequent season but we detected no longer-term effects. Neither species was affected by autumn precipitation, summer heat waves, or winter cold. Hurricane- and drought-induced changes in abundance of white-footed mice could affect their short-term interactions with prey (e.g., gypsy moth pupae, tree seeds, and songbird eggs) and parasites (e.g., ticks and zoonotic pathogens).

Estimating population densities of small mammals (< 100g) has typically been carried out by intensive livetrapping, but this technique may be stressful to animals and the effort required is considerable. Here, we used camera traps to detect small mammal presence and assessed if this provided a feasible alternative to livetrapping for density estimation. During 2010–2012, we used camera trapping in conjunction with mark–recapture livetrapping to estimate the density of northern red-backed voles ( Myodes rutilus ) and deer mice ( Peromyscus maniculatus ) in the boreal forest of Yukon, Canada. Densities for these 2 species ranged from 0.29 to 9.21 animals/ha and 0 to 5.90 animals/ha, respectively, over the course of this investigation. We determined if hit window—the length of time used to group consecutive videos together as single detections or “hits”—has an effect on the correlation between hit rate and population density. The relationship between hit rate and density was sensitive to hit window duration for Myodes with R2 values ranging from 0.45 to 0.59, with a 90-min hit window generating the highest value. This relationship was not sensitive to hit window duration for Peromyscus , with R2 values for the tested hit windows ranging from 0.81 to 0.84. Our results indicate that camera trapping may be a robust method for estimating density of small rodents in the boreal forest when the appropriate hit window duration is selected and that camera traps may be a useful tool for the study of small mammals in boreal forest habitat.

Across invertebrates and vertebrates, steroids are potent signaling molecules that affect nearly every cell in the organism, including cells of the nervous system. Historically, researchers have focused on the genomic (or "nuclear-initiated") effects of steroids. However, all classes of steroids also have rapid non-genomic (or "membrane-initiated") effects, although there is far less basic knowledge of these non-genomic effects. In particular, steroids synthesized in the brain ("neurosteroids") have genomic and non-genomic effects on behavior. Here, we review evidence that estradiol has rapid effects on aggression, an important social behavior, and on intracellular signaling cascades in relevant regions of the brain. In particular, we focus on studies of song sparrows (Melospiza melodia) and Peromyscus mice, in which estradiol has rapid behavioral effects under short photoperiods only. Furthermore, in captive Peromyscus, estrogenic compounds (THF-diols) in corncob bedding profoundly alter the rapid effects of estradiol. Environmental factors in the laboratory, such as photoperiod, diet, and bedding, are critical variables to consider in experimental design. These studies are consistent with the hypothesis that locally-produced steroids are more likely than systemic steroids to act via non-genomic mechanisms. Furthermore, these studies illustrate the dynamic balance between genomic and non-genomic signaling for estradiol, which is likely to be relevant for other steroids, behaviors, and species.

The evolutionary relationships between Peromyscus, Habromys, Isthmomys, Megadontomys, Neotomodon, Osgoodomys, and Podomys are poorly understood. In order to further explore the evolutionary boundaries of Peromyscus and compare potential taxonomic solutions for this diverse group and its relatives, we conducted phylogenetic analyses of DNA sequence data from alcohol dehydrogenase (Adh1-I2), beta fibrinogen (Fgb-I7), interphotoreceptor retinoid-binding protein (Rbp3), and cytochrome-b (Cytb). Phylogenetic analyses of mitochondrial and nuclear genes produced similar topologies although levels of nodal support varied. The best-supported topology was obtained by combining nuclear and mitochondrial sequences. No monophyletic Peromyscus clade was supported. Instead, support was found for a clade containing Habromys, Megadontomys, Neotomodon, Osgoodomys, Podomys, and Peromyscus suggesting paraphyly of Peromyscus and confirming previous observations. Our analyses indicated an early divergence of Isthmomys from Peromyscus (approximately 8 million years ago), whereas most other peromyscine taxa emerged within the last 6 million years. To recover a monophyletic taxonomy from Peromyscus and affiliated lineages, we detail 3 taxonomic options in which Habromys, Megadontomys, Neotomodon, Osgoodomys, and Podomys are retained as genera, subsumed as subgenera, or subsumed as species groups within Peromyscus. Each option presents distinct taxonomic challenges, and the appropriate taxonomy must reflect the substantial levels of morphological divergence that characterize this group while maintaining the monophyletic relationships obtained from genetic data.

DNA sequence and morphometric data were used to re-evaluate the taxonomy and systematics of Peromyscus pectoralis. Phylogenetic analyses (maximum likelihood and Bayesian inference) of DNA sequences from the mitochondrial cytochrome-b gene in 44 samples of P. pectoralis indicated 2 well-supported monophyletic clades. The 1st clade contained specimens from Texas historically assigned to P. p. laceianus; the 2nd was comprised of specimens previously referable to P. p. collinus, P. p. laceianus, and P. p. pectoralis obtained from northern and eastern Mexico. Levels of genetic variation (~7%) between these 2 clades indicated that the genetic divergence typically exceeded that reported for other species of Peromyscus. Samples of P. p. laceianus north and south of the Río Grande were not monophyletic. In addition, samples representing P. p. collinus and P. p. pectoralis formed 2 clades that differed genetically by 7.14%. Multivariate analyses of external and cranial measurements from 63 populations of P. pectoralis revealed 4 morpho-groups consistent with clades in the DNA sequence analysis: 1 from Texas and New Mexico assignable to P. p. laceianus; a 2nd from western and southern Mexico assignable to P. p. pectoralis; a 3rd from northern and central Mexico previously assigned to P. p. pectoralis but herein shown to represent an undescribed taxon; and a 4th from southeastern Mexico assignable to P. p. collinus. Based on the concordance of these results, populations from the United States are referred to as P. laceianus, whereas populations from Mexico are referred to as P. pectoralis (including some samples historically assigned to P. p. collinus, P. p. laceianus, and P. p. pectoralis). A new subspecies is described to represent populations south of the Río Grande in northern and central Mexico. Additional research is needed to discern if P. p. collinus warrants species recognition.

The complexity of a community can play a fundamental role in the prevalence of pathogens by altering interactions among hosts and pathogen transmission. Information on the frequency of contacts between individuals and the distribution of contact rates in a population is critical to predicting pathogen prevalence. However, contact rates are notoriously difficult to document especially in small, nocturnal species. We have been documenting the contact rates of deer mice (Peromyscus maniculatus) in nature with respect to infection with Sin Nombre virus (SNV), a zoonotic pathogen, and the biodiversity of the mammalian community. Our long-term field studies, as well as those of others, revealed that prevalence of SNV in deer mice is related to the complexity of the mammalian community such that pathogen prevalence is lower in more diverse communities. Using a combination of techniques, we found evidence that contact rates between deer mice differ with respect to biodiversity. Deer mice in more complex communities had fewer intraspecific interactions than those in less diverse communities. Contact rates of individual deer mice were highly variable with a minority of the deer mice accounting for a majority of the interactions. Infection with SNV was related to risk-taking behavior; animals categorized as “bold” were 3 times more likely to be infected than “shy” deer mice. Results of these studies have implications for pathogen management in wildlife and humans.

The use of standard external measurements is pervasive in mammalogy, due in part to their applicability to both living and dead animals. Nevertheless, comparisons of measurements made between animals in different pre- and postmortem body states may be problematic. To investigate the impact of body state on standard measurements and their associated variances, we took 5 external measurements on 70 Mus musculus while living and in 3 postmortem body states (primary flaccidity, rigor mortis, and secondary flaccidity). Total length, tail length, and hind-foot length were significantly longer in states of primary and secondary flaccidity than when measured on live individuals or those in rigor mortis. Ear length increased from living to primary flaccidity, after which it decreased. Weight decreased between each postmortem body state, likely as an artifact of desiccation. Variance was always greater for measurements taken in the living state than during postmortem body states. Irrespective of body state, variance was particularly high for ear length and hind-foot length, which are prone to observer bias and should be used with caution. Additional tests using field-collected data from populations of Peromyscus leucopus and Peromyscus maniculatus confirmed our lab-based results from M. musculus. External measurements taken during a postmortem state should not be compared to measurements from live animals and could lead to incorrect species identification. Additionally, comparisons among individuals measured in different postmortem body states may confound efforts to assess changes in body size over space or time.

Major challenges for illuminating the genetic basis of phenotypic evolution are to identify causative mutations, to quantify their functional effects, to trace their origins as new or preexisting variants, and to assess the manner in which segregating variation is transduced into species differences. Here, we report an experimental analysis of genetic variation in hemoglobin (Hb) function within and among species of Peromyscus mice that are native to different elevations. A multilocus survey of sequence variation in the duplicated HBA and HBB genes in Peromyscus maniculatus revealed that function-altering amino acid variants are widely shared among geographically disparate populations from different elevations, and numerous amino acid polymorphisms are also shared with closely related species. Variation in Hb-O2 affinity within and among populations of P. maniculatus is attributable to numerous amino acid mutations that have individually small effects. One especially surprising feature of the Hb polymorphism in P. maniculatus is that an appreciable fraction of functional standing variation in the two transcriptionally active HBA paralogs is attributable to recurrent gene conversion from a tandemly linked HBA pseudogene. Moreover, transpecific polymorphism in the duplicated HBA genes is not solely attributable to incomplete lineage sorting or introgressive hybridization; instead, it is mainly attributable to recurrent interparalog gene conversion that has occurred independently in different species. Partly as a result of concerted evolution between tandemly duplicated globin genes, the same amino acid changes that contribute to variation in Hb function within P. maniculatus also contribute to divergence in Hb function among different species of Peromyscus. In the case of function-altering Hb mutations in Peromyscus, there is no qualitative or quantitative distinction between segregating variants within species and fixed differences between species.

SYNOPSIS: Effective aerobic locomotion depends on adequate delivery of oxygen and an appropriate allocation of metabolic substrates. The use of metabolic substrates during exercise follows a predictive pattern of lipid and carbohydrate oxidation that is similar in lowland native cursorial mammals. We have found that in two highland lineages of mice (Phyllotis and Peromyscus) the fuel use pattern is shifted to a greater reliance on carbohydrates compared to their lowland conspecifics and congenerics. However, there is variation between lineages in the importance of phenotypic plasticity in the expression of this metabolic phenotype. Moreover, this metabolic phenotype is independent of running aerobic capacity and can also be independent of thermogenic capacity. For example, wild-caught mice from a highland population of deer mice (Peromyscus maniculatus) housed in warm normoxic laboratory conditions maintain higher maximum cold-induced oxygen consumption in acute hypoxia than lowland congenerics, but shivering and non-shivering thermogenesis is supported by high rates of lipid oxidation. This is reflected in the consistently higher activities of oxidative and fatty acid oxidation enzymes in the gastrocnemius of highland deer mice compared to lowlanders, which are resistant to hypoxia acclimation. While a fixed trait in muscle aerobic capacity may reflect the pervasive and unremitting low PO2 at high altitudes, muscle capacities for substrate oxidation may be more flexible to match appropriate substrate use with changing energetic demands. How shivering thermogenesis and locomotion potentially interact in the matching of muscle metabolic capacities to appropriate substrate use is unclear. Perhaps it is possible that shivering serves as "training" to ensure muscles have the capacity to support locomotion or visa-versa.

White-footed mouse (Peromyscus leucopus) consumption of plant fruits can significantly affect vegetative community structure. However, little is known about the effects of relative selection between different fruit species on the establishment and spread of invasive shrubs. Depending on foraging activity, white-footed mice may provide resistance against exotic establishment, have no impact, or even contribute to invasive spread. Selection of invasive Morrow's honeysuckle (Lonicera morrowii) and 5 native soft-mast species was studied across 3 cover types (forest, field, and edge) and 2 seasons in southwestern Pennsylvania. Foraging stations, containing equal quantities of each species, were randomly placed in each of the cover types. In addition, nutrient composition and total energy were measured. Fruit consumption was nonrandom based on compositional analysis (P < 0.05). Honeysuckle was consumed over native staghorn sumac (Rhus typhina); otherwise, it was consumed less than all natives. Honeysuc...

Historically, specimens representing the Peromyscus boylii species group (from montane regions of western and southwestern Mexico) have been referred to as P. boylii or P. levipes. However, previous studies indicated that specimens from eastern Nayarit possessed a karyotype and mitochondrial DNA haplotype distinct from other members of the P. boylii species group. Together, these data precluded an assignment of these specimens to any currently recognized taxon in the P. boylii species group. Availability of additional specimens from this region provided an opportunity to reevaluate the taxonomic status of this complex. Analyses of 18 morphological characters demonstrated that specimens from eastern Nayarit possessed a significantly longer length of the nasals relative to the other species. Further, phylogenetic analyses (parsimony and likelihood) of DNA sequences obtained from the mitochondrial cytochrome-b gene indicate these specimens form a monophyletic clade embedded within a strongly support...

Extreme weather variation on the northern Great Plains of North America can potentially influence the abundance of grassland rodents across vast areas. We used the remains of 33,697 small mammals collected from owl pellets in central and western Canada over 15 years to determine the influence of weather on the annual abundance of deer mice (Peromyscus maniculatus), sagebrush voles (Lemmiscus curtatus), and meadow voles (Microtus pennsylvanicus). Weather variation affected the annual abundances of all 3 species; however, influence on deer mouse and sagebrush vole annual abundances was relatively small compared to that on meadow voles. This finding may indicate that factors other than weather (i.e., habitat availability) are more important for the abundance of deer mice and sagebrush voles at the landscape scale. In contrast, meadow voles were positively associated with the duration of snow cover above the hiemal threshold (20 cm), exhibiting up to 5-fold increases (i.e., irruptions) in abundance f...

Seeds are an important food resource for many rodents, but the decision to consume or cache seeds when they are encountered can be influenced by numerous factors such as their abundance, nutritional value, and plant secondary compound (PSC) contents. Although previous studies on rodent foraging behaviors have focused on the effects of specific seed characteristics, the combined impact of seed quality and abundance is unclear. Here, we used artificial food patches in the field to examine the foraging behaviors of deer mice (Peromyscus maniculatus) in response to varying abundances of high– and low–nutritional-quality conifer seeds. We also used a variant of giving-up densities to assess mouse perception of the quality of seeds in patches. Mice treated white spruce (Picea glauca) seeds as a high-quality food source in the field, whereas subalpine fir (Abies lasiocarpa) seeds were treated as low quality, corresponding to their nutrient and PSC contents. Observations of foraging behaviors showed a st...

We used carbon and nitrogen isotopes measured in hair to compare the diets of 2 sympatric species of wild mice, Peromyscus californicus and P. boylii, in Santa Cruz County, California. The ability of these 2 Peromyscus species to coexist is thought to be the result of spatial partitioning through canopy plant associations as well as possible dietary niche partitioning. We used stable isotope analysis to determine the trophic level at which each species is feeding and stable isotope mixing models to estimate dietary contributions of various arthropod and plant-derived food sources. We found P. californicus to be omnivorous, specializing mainly on arthropods and consistently feeding at a higher trophic level than P. boylii. P. boylii is omnivorous as well, but specializes mainly on tanoak (Notholithocarpus densiflorus) acorns. Dietary niche partitioning appears to be seasonal; in the fall, partitioning breaks down to some degree, likely because food is so abundantly available, and both species consume a larger, overlapping array of acorns and arthropods. These findings coupled with other studies on habitat niche partitioning present a clearer picture of how these 2 sympatric species can coexist.

Little is known about the macrohabitat associations of rodents and shrews in prairie landscapes because of the logistic constraints of conventional trapping. We used the remains of 60,972 small mammals in owl pellets to assess factors affecting small mammal composition across 4.3 million hectares of the northern Great Plains of North America. Cropland with clay soils was dominated by deer mice (Peromyscus maniculatus), whereas areas with higher proportions of native grassland and moderately sandy soils supported communities with more sagebrush voles (Lemmiscus curtatus). Areas with clay soils and higher annual precipitation were associated with higher proportions of house mice (Mus musculus), meadow voles (Microtus pennsylvanicus), and shrews (Blarina brevicauda and Sorex species), whereas drier areas with sandier soils and lower annual precipitation were dominated by olive-backed pocket mice (Perognathus fasciatus) and northern grasshopper mice (Onychomys leucogaster). Contrary to extrapolations...

Based on examination of 705 museum specimens, including the holotype and individuals identified using molecular attributes, we characterized Schmidly's deer mouse (Peromyscus schmidlyi) using a set of morphometric and morphological characteristics. Discriminant function analysis was performed on 14 cranial attributes to find those most useful to separate species. Factorial analysis of variance and multivariate analysis of variance were used to test for significant differences due to species, sex, age, and geographic location. We compared and contrasted P. schmidlyi with P. boylii rowleyi and P. spicilegus, morphologically similar and partially sympatric species, and reevaluated the distribution of the 3 species in the Sierra Madre Occidental (SMO) of Mexico. P. schmidlyi is a medium-sized Peromyscus distinguishable by a set of characters including overall size intermediate between P. spicilegus and P. boylii rowleyi, relatively wider braincase and zygoma, hourglass-shaped interorbital constrictio...

Oak-dominated forest has declined in the eastern United States as shade-tolerant mesophytic species (e.g., maple [Acer spp.]) replace oaks (Quercus spp.), sparking concern among ecologists regarding species that consume acorns. Our goal was to describe how increasing mesophication of oak forests may affect consumers in higher trophic levels. We investigated relationships among forest composition, mast production, small-mammal density, and carnivore occurrence in 8 stands representing a gradient of oak–hickory dominance in central hardwood forest in southern Illinois. We livetrapped small mammals for >24,000 trap-nights in June–August 2009–2011 with trapping webs to estimate population density of mice (Peromyscus spp.). We collected mast seeds during October–November 2009–2010 and calculated average dry biomass (g/m2) for each species and stand. During winters 2009–2011, we photographed carnivores using baited camera traps. We regressed mast biomass on measures of forest composition and regressed ...

Rattlesnakes use chemical stimuli in ambush site selection and for relocation of envenomated prey through strike-induced chemosensory searching. Shifts in responsiveness to prey chemicals have been documented in many snakes, and often correlate with prey commonly taken as snakes increase in age and size as well as geographical locations of the species. For instance, neonate rattlesnakes that prey primarily on ectotherms responded most strongly to chemical cues of commonly taken lizard prey, whereas adult rattlesnakes that prey primarily on small mammals responded significantly to chemical cues of com- monly taken rodents. In the current study, 11 Prairie Rattlesnakes Crotalus viridis viridis which were classified as large neonates based on measures of snout-vent length (SVL) and body mass, yet chronologically were at or near adulthood, were tested for their responsiveness to chemical extracts of natural and non-natural prey items. Although the snakes had eaten only neonate lab mice (Mus musculus), they responded significantly more to chemical cues of natural prey items and particularly to chemical cues of prey normally taken by subadults (Peromyscus mice and Sceloporus lizard). These results suggest that ontogenetic shifts in re- sponsiveness to natural prey chemical cues are innately programmed and are not based on body size or feeding experience in C. v. viridis. This does not imply, however, that growth and experience are without effects, especially with novel prey or rare prey that have experienced recent population expansion (Current Zoology 59 (2): 175−179, 2013). Keywords Chemical cues, Chemoreception, Crotalus viridis viridis, Prey choice, Vomeronasal

The etiological agents that cause human babesiosis (Babesia microti) and Lyme disease (Borrelia burgdorferi) share a common tick vector (Ixodes scapularis Say) and rodent reservoir (Peromyscus leucopus), but because the geographical distribution of babesiosis is more restricted than Lyme disease, it was not considered a nationally notifiable disease until 2011. Although recent studies have shown dramatic increases in the number of cases of babesiosis and expansion of its range, little is known about infection and coinfection prevalence of these pathogens in the primary tick vector. Of the 478 I. scapularis nymphs collected within six Monmouth County, NJ, municipalities between 2004 and 2006, 4.0 and 10.0% were infected with B. microti and B. burgdorferi, respectively, while 2.9% were coinfected. Analysis of the 610 I. scapularis adults collected during the same period yielded an infection prevalence of 8.2% for B. microti and 45.2% for B. burgdorferi, while 6.2% were coinfected. The potential public health importance of these findings is discussed.

Rodents make foraging decisions by balancing demands to acquire energy and mates with the need to avoid predators. To identify variations in the risk of predation, nocturnal rodents may use moonlight as a cue of risk. Moonlight avoidance behaviors have been observed in many nocturnal rodent species and are widely generalized to small mammals. However, most prior studies have been limited to 1 species or 1 study site, or occurred in modified habitats. We evaluated desert rodent activity patterns in natural habitats from 1999 to 2006 at 62 study sites across the Great Basin Desert of western North America. Rodent activity was examined by livetrapping in open habitats, using the presence of the sand-obligate kangaroo mouse (Microdipodops) as a habitat indicator. Activity patterns were assessed on 69 nights with clear skies and compared to corresponding moonlight values (moon phase and brightness) to evaluate the frequency of moonlight avoidance. Analyses of total activity of all species in the rodent assemblage relative to moonlight showed a distinct nonrandom (triangular-shaped) pattern but no significant correlations. However, individual genera of desert rodents responded differently to moonlight. Only kangaroo rats (Dipodomys) displayed significant moonlight avoidance patterns; they were maximally active at significantly different moonlight levels and avoided bright moonlight to a greater extent than co-occurring rodents. Moonlight seemed to limit the activity of kangaroo rats most strongly on bright nights during waxing moon phases and summer seasons, but not significantly during the spring or fall seasons, or during waning moons. Rather than avoiding moonlight, the activity of deer mice (Peromyscus), pocket mice (Perognathus), and kangaroo mice may be governed by changes in competition with kangaroo rats. Differences in the body size, locomotion, and space use of kangaroo rats relative to other rodents may explain why different moonlight responses were detected, especially if these traits alter how rodents perceive risk from bright moonlight. These findings indicate that moonlight avoidance may be a specialized trait of kangaroo rats rather than a general behavior of nocturnal desert rodents in the Great Basin.

Golden mice (Ochrotomys nuttalli) occur throughout the southeastern United States, and are on the periphery of their geographic range in southern Illinois, where they are a state-listed threatened species. We assessed relative abundance and distribution of populations of golden mice in southern Illinois by mark–recapture and occupancy models, and throughout the core geographic distribution to determine if they should be removed from threatened status (delisted). We also tested the “abundant-center” hypothesis that states that occurrence and abundance of a species is greater in the core of the range than at the periphery. We captured golden mice in 21 of 24 sites in southern Illinois, but only 13 of 24 sites in the core of the range. The total number of individual golden mice captured in southern Illinois (n = 99) was 3.3 times greater (χ21 = 36.91, P < 0.001) than in the core (n = 30). Habitat factors at sites (mean number of climbing vines, woody stems, and herbaceous stems) affected occupancy o...

Snakes often have specialized diets that undergo a shift from one prey type to another depending on the life stage of the snake. Crotalus viridis viridis (prairie rattlesnake) takes different prey at different life stages, and neonates typically prey on ectotherms, while adults feed almost entirely on small endotherms. We hypothesized that elevated rates of tongue flicking to chemical stimuli should correlate with particular prey consumed, and that this response shifts from one prey type to another as in- dividuals age. To examine if an ontogenetic shift in response to chemical cues occurred, we recorded the rate of tongue flicking for 25 neonate, 20 subadult, and 20 adult (average SVL = 280.9, 552, 789.5 mm, respectively) wild-caught C. v. viridis to chemi- cal stimuli presented on a cotton-tipped applicator; water-soluble cues from two ectotherms (prairie lizard, Sceloporus undulatus, and house gecko, Hemidactylus frenatus), two endotherms (deer mouse, Peromyscus maniculatus and lab mouse, Mus musculus), and water controls were used. Neonates tongue flicked significantly more to chemical cues of their common prey, S. undulatus, than to all other chemical cues; however, the response to this lizard's chemical cues decreased in adult rattlesnakes. Subadults tongue flicked with a higher rate of tongue flicking to both S. undulatus and P. maniculatus than to all other treatments, and adults tongue flicked significantly more to P. maniculatus than to all other chemical cues. In addition, all three sub-classes demonstrated a greater response for natural prey chemical cues over chemical stimuli of prey not encountered in the wild (M. musculus and H. frenatus). This shift in chemosensory response correlated with the previously described ontogenetic shifts in C. v. viridis diet. Be- cause many vipers show a similar ontogenetic shift in diet and venom composition, we suggest that this shift in prey cue dis- crimination is likely a general phenomenon among viperid snakes (Current Zoology 58 (4): 549-555, 2012). Keywords Gape-limited predator, Prey choice, Trophic adaptation, Vomeronasal chemoreception

Extent of edge habitat is often defined arbitrarily from a boundary between 2 habitats. Instead, we assessed whether space use by white-footed mice (Peromyscus leucopus) responded to habitat changes along an edgeto-interior gradient in eastern deciduous forest. We measured movement distance as mean squared distance from center of activity (MSD), microhabitat variables, and distance to nearest conspecific neighbor. Distance of center of activity from the habitat edge was not affected by animal age (e.g., body mass). Male MSD did not change along the habitat gradient. Female MSD increased with distance from habitat edge, was negatively related to vertical cover, and was positively related to diameter at breast height of trees. Increased MSD in forest interior may reflect lower availability of nesting sites or diet items. Decreased MSD at forest edge may reflect higher vegetative cover that provided more food or behaviors by mice to avoid edge predators/ parasites. Female use of vertical habitat increased with distance from habitat edge, and was related to fewer trees, less woody debris, and smaller distance to nearest conspecific neighbor. Fewer trees and less woody debris could negatively affect availability of nesting sites, travel routes, or diet items, hence greater use of vertical habitat. The nearest-neighbor effect may indicate intraspecific competition from lower availability of resources in forest interiors. Such responses in space use along a habitat gradient indicated that defining edge habitat as an arbitrary distance from a habitat edge may be insufficient to understand ecology and behavior of small mammals.

Data collected over the past 15 years in Colorado show that deer mice (Peromyscus maniculatus) collected in southeastern Colorado have a significantly lower Sin Nombre virus (SNV) antibody prevalence than mice from western Colorado. Based on mitochondrial sequences, P. maniculatus was recently subdivided into 6 clades. Clade 1a occurs throughout the mountainous regions of western Colorado and clade 2 crosses the short-grass steppe in southeastern Colorado. We used mitochondrial single nucleotide polymorphisms (SNPs) from this earlier study and 14 nuclear SNPs to test the hypothesis that clade 2 mice consist of a separate subspecies. Little genetic differentiation was detected (FST 5 0.027) among 435 deer mice collected from 8 locations in western Colorado. In contrast, 171 deer mice collected from 4 locations in eastern Colorado were genetically differentiated from one another (FST 5 0.168) and from those captured in western Colorado (FST 5 0.256). During this survey we identified a single locality in central Colorado where both clade 1a and 2 mice are sympatric and have a high SNV seropositivity (52.2% in 2006; 24.4% in 2009). Antibody prevalence was 21.1% among clade 2 mice, lower than the 32.4% rate among clade 1a mice, suggesting that clade 2 mice may, for both genetic and environmental reasons, have lower susceptibility to SNV infection.

Many species of wild mammals occur in habitats that have been disturbed by fragmentation or degraded in quality. Previous researchers have hypothesized that demographic characteristics of populations may shift with changes in environmental conditions, with self-regulatory ability increasing with environmental suitability. We studied responses of white-footed mice (Peromyscus leucopus) to habitat disturbance. Given that optimal habitat for this species is deciduous woodland, we predicted that populations in habitats disturbed by cutting woody vegetation would be lower and more variable in density than in undisturbed habitat, density and stability of populations in disturbed habitat would increase over time, survival would be higher in undisturbed than in disturbed habitat, and populations in undisturbed habitat would show a greater degree of self-regulation. This 6year study in western Pennsylvania involved 3 replicated study sites (each 3.8 ha), with woody vegetation removed on half of each site prior to beginning the study. Density in disturbed treatment averaged 65% of density in undisturbed habitat. There were no differences between treatments in survival or in population growth rates over time. Population trends over time were similar between treatments, populations in disturbed habitat did not become more stable with time, and density did not converge with that of undisturbed habitat. Although populations in the undisturbed habitat were clearly self-regulating, those in disturbed habitats were not. Despite expectations that demographic performance will align with environmental suitability, it may be difficult to ascribe a particular demography to a habitat generalist such as P. leucopus.

Food resources and predation play important roles in determining small mammal population dynamics. These factors also can interact as individuals under predation pressure make trade-offs between access to food resources and exposure to predators. Fires consume food sources and reduce cover, which increases exposure to predators. For species that occur in areas with frequent fire, it is instructive to consider how all of these factors interact to affect populations of interest. We examined how supplemental feeding, mammalian predator exclusion, and prescribed fire affected survival, abundance, and reproduction of cotton mice (Peromyscus gossypinus) and oldfield mice (P. polionotus) in a longleaf pine ecosystem. Burning and predator exclusion interacted to affect survival of cotton mice; survival was similar in predator exclosures and controls in nonburning periods and in controls following prescribed burns but increased in exclosures following burns. Rates of transitions to reproductive states (wh...